Method of reducing notch sensitivity in tubular products

ABSTRACT

A method of reducing notch sensitivity in tubular products, to inhibit premature bursting on such being subjected to a high internal pressure. An interior circumferential region is uniformly roughened in the product together with removal of some material in the wall of the product. Such effectively eliminates notches as stress risers, and on a high internal pressure condition occurring, resultant stress is distributed throughout the uniformly roughened region.

United States Patent Baksay METHOD OF REDUCING NOTCH SENSITIVITY INTUBULAR PRODUCTS [75]' Inventor:

[73] Assignee: 'Zirconium Technology Corporation,

Albany, Oreg.

22 Filed: Jan. 17, 1972 21' Appl. No.: 218,472

Related US. Application Data [63] ContinuatiOn-in-part of Ser. NO.191,201, Oct. 21,

Ivan Baksay, Albany, Oreg.

52 Us. (:1 ..s1/319,51/290,51/323 51 1m.c1 B24b 1/00 [58] FieldofSearch..51/319,320,321,290, 7 51/323; 252/142-151 [56] References Cited UNITEDSTATES PATENTS 3,549,735 12/1970 Moss 51/319 X Apr. 23, 1974 2,373,4594/1945 Colwell 51/290 2,087,694 7/1937 7 Malmros. 51/320 X 1,610,92712/1926 Butler 51/319 Primary Examiner-Donald G. Kelly Attorney, Agent,or Firm-Kolisch, Hartwell &

Dickinson [57] ABSTRACT A method Of reducing notch sensitivity intubular products, to inhibit premature bursting On such being subjectedto a high internal pressure. An interior circumferential region isuniformly roughened in the product together with removal of somematerial in the wall of the product. Such effectively eliminates notchesas stress risers, and on a high internal pressure condition occurring,resultant stress is distributed throughout the uniformly roughenedregion.

3 Claims, 2 Drawing Figures METHOD OF REDUCING NOTCH SENSITIVITY INTUBULAR PRODUCTS This is a continuation-in-part of my prior-filedapplication entitled, Method of Reducing Notch Sensitivity in TubularProducts, filed Oct. 21, 1971, Ser. No. 191,201. I

This invention relates to a method of reducing notch sensitivity intubular products.

In the manufacture of precisionly dimensioned seamless tubing, it isconventional to prepare a so-called tube hollow, usually through anextrusion process, and to pass such tube hollow through a tube reduceror other means whereby tube wall area reduction takes place. Thisreduction in tube wall area is produced both by reducing the diameter ofthe tube, as well as by reducing the thickness of the tube wall.

It is known that extrusion tool marks may leave longitudinal seams onthe inner circumferential surface of a tube hollow. During subsequentreducing operations, these may be folded up into lap type defects thatfor the most part may be detected using non-destructive testingtechniques, such as ultrasonic testing. By way of illustration,ultrasonic testing is capable of detecting a notch where such has adepth exceeding about 2 mils or more.

It is also known that pits and other types of discontinuities (embracedwithin the meaning of notch as used herein), not necessarily coming fromthe tube hollow, may also be present on the inner circumference of thefinished tube.

Discontinuites of the type discussed may be of such small size that theyare not detectable using nondesctructive testing prodecures. However,they introduce what is referred to herein as notch sensitivity, sincetheir presence in the tube has been found to reduce seriously thecircumferential ductility or fatique resistance of the product. Thesesmall surface discontinuiti'es function as stress risers, whereby ondestructive testing for such things as circumferential ductility (bursttest), or for fatique resistance (pressure impulse test, etc.),premature failure is noted in the tube even though the product in allother respects has the appearance of meeting requisite standards.

The possession of a high degree of circumferential ductility is arequirement in certain types of tubing which may be expected during itsservice life to be subjected to a high internal pressure condition.Exemplary of such an application is the zirconium alloy tubing used ascladding for reactor fuel. Fuel swelling and the formation of gaseousfission products may produce a volume expansion within the tubing, andwithout circumferential ductility such results in bursting andradioactive contamination of the reactor system. With other types oftubes, such as tubes annealed to habe highstrength levels, a high degreeof fatique resistance, as determined :by such tests as a pressureimpulse test, is an important property. Exemplary of this type of tubingis the titanium alloy tubes that have been used extensively forhydraulic pressure lines in aircraft and the like. Over a period oftime, sudden pressure surges, or accidental over pressures, producefailure in such lines if stress risers are present. By noting thesespecific areas of tube use, however, it is not intended to limit thisinvention to these particular uses.

A general object of this invention, therefore, it to provide a novelmethod of reducing notch sensitivity in tubular products.

More specifically, it is an object to provide such a method which isparticularly applicable to the processing of tubing possessing smalldiscontinuities which are ordinarily nondetectable using conventionalultrasonic, or other nondestructive techniques.

A more specific and further object of the invention is to provide such amethod wherein steps in the method comprise uniformly roughening aninterior circumferential region in the tubular product, and subsequentlydistributing stress within the product on a high internal pressurecondition occuring.

A further and more specific object of the invention is provision of sucha method wherein roughening is performed with reducing of tube wallthickness, wherein defect depth is reduced simultaneously with theintroduction of a roughened surface.

These and other objects and advantages of the invention will become morefully apparent as the following description is read in conjunction withthe accompanying drawings, wherein: Y 1

FIG. 1 is a cross section of a tube, on an enlarged scale, showing notchdefects on the interior surface thereof; and

FIG/2 is a cross section of such tube showing the tube after it has beenroughened according to the present invention.

Describing an embodiment of the invention, aZircaloy tube such as mightbe used to clad reactor, fuel in a nuclear reactor conventionally mayhave an ID. of approximately 1% inch, and a wall thickness ranging from20 to 50 mils. Such may be prepared from a tube hollow by passing tubehollow in multiple passes through a tube reducer or other machine.Typically, for instance, the original tube hollow may have an CD. ofapproximately 2 inches and a wall thickness of about 7% inch. During themultiple passes through the tube reducer, tube wall area reduction takesplace, both by reason of the diameter'of the tube hollow being reduced,and also through a reduction in wall thickness.

Finally prepared tubing of the type described may havean inner wallsurface which gives the appearance of being perfectly smooth and free ofany notches such as pits or other discontinuities. The tube may besubjected to ultrasonic tests which senses discontinuities of about twomil depth or more, without any such discontinuities being detected.However, when different specimens of such a tube are subjected todestructive testing, i.e., a burst test, a pressure impulse test, etc.to determine circumferential ductility or fatigue resistance, ratherinconsistant results may be obtained.

1 Typically, circumferential ductility is determined in a burst test bycalculating the total circumferential elongation occuring in thespecimen beforebursting, as .a percent of the original outercircumference of the specimen, such calculation being referred to aspercent TCE. Zircaloy tubing of the type described with notultrasonically detectable defects, and without treatment as contemplatedby the invention, may exhibit widely varying TCE values.

According to the invention, as interiorcircumferential region of thetube is uniformly roughened. With dis- .continuites of the typementioned, i.e., having a depth of less than about 2 .mils, theroughening imparted to the inside of the tube optimumly should liewithin the range of about 20 to 50 rms. Further, the roughening shouldbe uniform, in that from visual observation the roughening appearscontinuous over the inside of the tube.

In a preferred embodiment of the invention, roughening is performed byblasting the inside of the tube with particles of an abrasive material.Such blasting it effective not only to impart a uniform roughness to thetubes interior, but also to reduce to some extent the tube wallthickness. The tubes with which I have been concerned have ranged inlength from 5 to 25 feet. In roughening such tubes by blasting with anabrasive, the blast nozzle is introduced to each of the opposite ends ofthe tube. The abrasive airborne particles are directed by the nozzle ina turbulent type of flow down the interior of the tube to the tubesopposite open end. With blasting done from both ends, wall reductionthroughout the entire length of the tube is substantially uniform.

Ordinarily, and with defects of about two mil penetration or less, thetube wall thickness is reduced at least about one third of suchpenetration. For instance, with defects expected to have a depth rangeof up to about two mils, material removal is performed to reduce thewall thickness on an average at least about one third of this amount andpreferably about one half, which amounts to a wall reduction of one mil.

Ordinarily the amount of material removed during the blasting is lessthan about 5 percent of wall thickness. With tubes having a wallthickness greater than the range indicated, it is permissible to removemore material from the inside of the tube without exceeding thislimitation, as long as specified tolerances on wall thickness aremaintained.

In selecting the abrasive material used to roughen the tube interior,and to attain a roughness within the range indicated, the abrasivepreferably should have a particle size within the range of about 40 to150 mesh. An excellent abrasive to perform the roughening is siliconcarbide, although it should be obvious, of course, that any of the wellknown abrasive materials would be suitable.

Describing a particular roughening operation, Zircaloy alloy tubes of0.450 inch LD. and free of ultrasonically detectable discontinuitieswere roughened on their inner surfaces using a silicon carbide abrasive.The tubes .were blasted from both ends, using a conventional airpressure operated blasting nozzle inserted into each of opposite ends ofthe tubes. Air at 50 psi. was used in the blasting, and blasting wereperformed for a period of about 1 minute at each of the respective endsof the tubes. The blasting produced material removal from the innerwalls, whereby there was an average descrease in wall thickness of aboutone mil, plus or minus 0.3 mils. Surface roughness within the interiorof the tube was determined to be within the range of about 30 to 40 RMS,and appeared uniform throughout.

Tubes so processed were evaulated in the laboratory for circumferentialductility together with specimens of untreated tubes. In suchevaluation, the tubes were subjected to an internal pressure increasedto a level sufficient to produce bursting. Percent TCE was calculatedfor the various untreaded specimens, as well as the tube specimens whichhad been roughened as described. The untreated specimens exhibited apercent TCE ranging from about 8 to 23, with the average percent TCElying about midway between this range, or being approximately 15percent. With the treated specimens, calculated percent TCE ranged fromabout 22 to 25 percent.

In other tests, titanium alloy tubes annealed to have high strength weresimilarly roughened. These tubes were subjected to pressure impulsetests to determine fatigue resistance. Essentially all tubes tested metcommercial standards for aircraft hydraulic lines.

As illustrated in the accompanying drawings, a portion of a tube on agreatly enlarged scale is shown at 10. Such has an inner circumferentialsurface designated 10a. Notches in the surface are depicted at l2, l4,and 16. These notches, as is typical, vary in depth with notch 12 havinga depth of less than one mil, notch 14 having a depth of about 1 mi],and notch 16 having a depth of almost 2 mils. All of such notchedordinarily would be nondetectable using nondestructive testingtechniques.

With roughening of the tube interior, the tube appears as shown in Fig.2. The roughened surface is uniformly roughened as indicated by theoverlapping nature of the surfaces defining the depressions formed bythe abrasive particles. With material removal reducing wall thickness byabout 1 mil, the notches l2 and 14 are essentially completely removed. Atrace of notch 16 remains, but metal around the edge of the notch hasbeen removed, and because of this and, because the depth of the notch isdecreased by the metal removal, the notch in effect becomes lost amongstthe various depression produced by the roughening.

In some instances, to obtain desired cleanliness within the tube, it maybe desirable to subject the tube after the roughening operationdescribed to a pickling step which has the effect of removing someadditional material from the inner surface of the tube wall. Describinga typical pickling operation, the tube may be immersed in anacidpickling solution consisting of 15 parts by volume of a 4 percent byweight hydrofluoric acid solution, and parts by volume water. Exposureto the acid solution may not be long, usually not exceeding about a halfa minute. After the acid treatment, the tube is subjected to a waterrinse. Such a pickling removes approximately 0.5 mil of material fromthe surfaces exposed to the acid solution.

Theappearance of the tube interior after pickling indiscates asmoothening of the tube interior by the pickling step. Thus the RMS isreduced somethat through this cleaning operation. Tubes which have beencleaned by pickling, after having been roughened as contemplated by thisinvention, exhibit the same improvement of reduction to notchsensitivity as tubes that are roughened without pickling.

It should be pointed out here that tubes that are subjected to apickling, without roughening as by blasting, do not exhibit the sameimprovement with respect to reduced notch sensitivity. This maybeexplained by the fact that blasting, in a manner of speaking, ispreferential, in that it removes material along the plane of the surfaceon which it is performed without removing'material, for instance, fromthe base of a notch. Pickling, on the other hand, is nonpreferential, inthat when such is performed it removes material both from the plane ofthe surface as well as from the notch base.

In reducing the tube wall area of the tube hollow to produce a tube,when a tube reducer is utilized, the hollow is pushed through diescontacting the hollows outer surface which are in motion with theinternal area of the hollow supported on a stationary mandril. In aswaging operation, on the other hand, the tube hollow is pushed througha stationary die contacting the ho]- lows outer surface, together with astationary mandril which supports the inner surface of the tube hollow.So-called drawing is similar to swaging, save that the tube is pulledthrough the die and over the mandril, rather than pushed. In all thesetypes of reduction operations, it is normal to expect a somewhatsmoother outer surface of the tube, with less notch defects, than theinner surface, by reason of better lubrication during the reductionprocess.

The outer surface of the tube may be conditioned in a number ofdifferent ways to inhibit notch sensitivity therein. It should bepointed out, however, that the conditioning of the outer surface of atube does not present the same problems as conditioning the tubes innersurface, by reason of the fact that the surface is exposed, and visually(aided by magnifiying means if necsssary), it is possible todetermine-the extent, distribution, and nature of any notchestherealong.

In conditioning the outer surface of a tube, such may be done, forinstance, through a polishing action using polishing equipment, whichmight typically remove about a mil of material. This may be followed bya pickling action, carried on to remove about two mils of material fromthe outer surface. Polishing alone is not satisfactory, ordinarily, andmust be followed by a pickling, in order that any polishing marks beremoved.

In another type of conditioning of the outer surface, the tube, forinstance, may have its outer surface first detergent cleaned, thenpolished, followed by a pickling action, with a final peening of thesurface. The peening produces a working of the surface but does notresult in a material removal, as does the roughening action contemplatedfor the tube material by the present invention.

In performing roughening on an inner surface of a tube with an abrasiveand by blasting, as described above, the nozzle ejects the abrasiveparticles at an acute angle against the inner surface of the tube, whichparticles then move in a turbulent motion down through the tube interiorto produce the roughening desired. The process is particularly suitablefor a tube interior, since the particles are funneled down through thetube by the tube wall in random motion, to produce a material removalwhich is uniform about the circumference of the tube. Material removableon the outer surface of the tube through a blasting operation is not aseffectively performed, since the outer part of the tube does not operateto guide or channel abrasive particles, and as a consequence, uniformremoval circumferentially about the tube through the abrasive action isnot readily possible. According to the invention as preferred,therefore, it is contemplated that the tube outer surface be conditionedthrough steps such as polishing and pickling, polishing and peening, orcleaning, polishing, pickling and peening, carried on in such order,with the inner surface of the tube conditioned through the abrasiveroughening described.

The abrasive roughening is disti'nguishabled from the type of processused on the outside of the tube in that, in one operation, there is bothmaterial removal and condition of the tube surface, the removal in amanner of speaking being preferential. Polishing, for example, performsa preferential material removal, but must be followed by some sort ofconditioning step. Comparing a peening operation, this conditions thesurface'of a tube, but does not result in material removal.

The effect of the roughening, as contemplated by the invention, is toeliminate surface discontinuities in the tubes as stress risers. Stressbecomes distributed throughout the roughened internal regions of thetube on a high pressure occurring.

While an embodiment of the invention has been described it should beobvious that modifications and variations are possible without departingtherefrom.

I claim:

1. A method of processing a tubular product to inhibit bursting of theproduct on its being subjected to a high internal pressure condition,the method comprising preparing the tubular product with a relativelysmooth internal circumferential region with any surface discontinuitiestherein having a penetration into the wall of the product limited toless than about2 mils,

blasting with particles of abrasive the internal circumferential regionof the tubular product to obtain over such region a surface roughnesswithin the range of about 20 to 50 RMS, and

with such blasting removing material to reduce average wall thickness atleast about one third of the indicated limit of surface penetration, themethod producing a product operable, with a high internal pressurecondition occurring which stresses the product, to produce distributionof stress throughout the uniformly roughened internal circumferentialregion of the product.

2. The method of claim 1 wherein the blasting is performed from oppositeends of the tubular product to obtain a substantially uniform reductionin wall thickness throughout the entire length of the product.

3. The method of claim 1 which further comprises the step of picklingthe internal circumferential region of the tubular product performedafter blasting with particles of abrasive.

1. A method of processing a tubular product to inhibit bursting of theproduct on its being subjected to a high internal pressure condition,the method comprising preparing the tubular product with a relativelysmooth internal circumferential region with any surface discontinuitiestherein having a penetration into the wall of the product limited toless than about 2 mils, blasting with particles of abrasive the internalcircumferential region of the tubular product to obtain over such regiona surface roughness within the range of about 20 to 50 RMS, and withsuch blasting removing material to reduce average wall thickness atleast about one third of the indicated limit of surface penetration, themethod producing a product operable, with a high internal pressurecondition occurring which stresses the product, to produce distributionof stress throughout the uniformly roughened internal circumferentialregion of the product.
 2. The method of claim 1 wherein the blasting isperformed from opposite ends of the tubular product tO obtain asubstantially uniform reduction in wall thickness throughout the entirelength of the product.
 3. The method of claim 1 which further comprisesthe step of pickling the internal circumferential region of the tubularproduct performed after blasting with particles of abrasive.